1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including thin film transistors (hereinafter, TFT) and a method for manufacturing the same. Specifically, the present invention relates to electronic devices incorporating an electro-optical device typified by a liquid crystal display panel, or a light emitting display device having an organic light emitting element, as a component. In addition, the present invention relates also to a semiconductor device having a circuit including thin film transistors which are formed by transferring and bonding a layer which has been separated by a separation method, to a substrate.
It should be noted that the term “a semiconductor device” in this specification denotes all kinds of devices that can function by utilizing semiconducting properties, and includes all of an electro-optical device, a semiconductor circuit, and electronics.
2. Description of the Related Art
In recent years, a technique for manufacturing a thin film transistor by using a semiconductor thin film (from several nm to several hundred nm in thickness) that is formed over a substrate having an insulating surface has attracted a lot of attention. Such a thin film transistor is applied to an electronic device such as an IC or an electro-optical device widely, and developed intensively as a switching element of a picture display device in particular.
Various applications using such a picture display device have been expected, and particularly, application to a portable device attracts attention. As a substrate for forming these TFTs, a glass substrate or a quartz substrate is widely used now, however, these substrates have some drawbacks such as being fragile and heavy. Further, these substrates are unsuitable for mass-production since it is difficult to use a large-sized glass substrate or a large-sized quartz substrate. Therefore it has been attempted that an element including TFTs is formed over a substrate having flexibility as typified by a flexible plastic film.
However, the maximum temperature of the process should be lowered since the heat resistance of a plastic film is low. As a result, it has been impossible to form a TFT having as good characteristics as that formed over a glass substrate. Thus, a high-performance liquid crystal display device or light emitting element using a plastic film has not been realized yet.
There has been proposed a method of separating from the substrate a layer to be separated that exists with a separation layer therebetween. For example, the technique stated in Patent Document 1 (Japanese Published Unexamined Application No. Hei 10-125929) is that a separation layer containing amorphous silicon (or, polysilicon) is provided, the amorphous silicon is irradiated with laser light through the substrate, to diffuse hydrogen included in the amorphous silicon, thereby producing a space to separate the substrate. In addition, in Patent Document 2 (Japanese Published Unexamined Application No. Hei 10-125930), a liquid crystal display device is completed by bonding a separated layer (a layer to be transferred in Patent Document 2) to a plastic film, by using the technique of Patent Document 1.